Large quantities of inert gas, particularly argon and above all nitrogen, are used for example in the pharmaceutical and electronic industries, particularly during fabrication of semiconductors. Above all in this latter case, these inert gases must be as pure as possible, and, particularly, they must be substantially free from impurities such as carbon monoxide and hydrogen, which reduce the quality and performances of the semiconductors. Until recently, contents of carbon monoxide and/or hydrogen of the order of several hundreds of ppb (parts per billion by volume) were still tolerated.
But at present, the electronic industry requires inert gases, such as nitrogen, of high purity, which is to say containing less than about 10 ppb of hydrogen and less than 5 ppb of carbon monoxide.
So as to prepare high purity nitrogen, there has already been proposed by the applicant, in European patent application EP-A-0 454 531, a process according to which there is eliminated from air, the impurities carbon monoxide and/or hydrogen, by passage of this air over a catalyst constituted of particles of at least one metallic element selected from the group consisting of copper and metals of the platinum family, namely ruthenium, rhodium, palladium, osmium, iridium and platinum, these particles being supported on a support having a large specific surface. The supports can consist of zeolites, silica or alumina. Air treated by passage over the catalyst is, most often, air compressed by means of a compressor; at the output of this compressor, the air has a temperature usually comprised between 80.degree. C. and 120.degree. C. The air thus purified is stripped of the water and carbon dioxide that it contains, for example by adsorption on a molecular sieve. After which, the air is sent to a distillation column for the separation of nitrogen from the other constituents of the air, essentially oxygen. The applicant has found that good results have been obtained by means of this process, in particular when the catalysts comprise large quantities of particles such as palladium and platinum. However, these precious metals are of high cost, which increases too much the overall cost of preparation of high purity nitrogen.
Moreover, it has been found by the applicant that the catalysts described in the patent EP-A-0 454 531 must, for effective elimination of carbon monoxide and hydrogen, either be frequently regenerated, for example twice a day, or be placed in contact with compressed air whose temperature is brought to about 200.degree. C.
The frequent regeneration of the catalyst involves using two catalyst beds in parallel and operating alternately, one permitting purification of the air while the other is regenerated. As indicated above, compressed air leaves the compressor at a temperature usually comprised between 80.degree. C. and 120.degree. C. Means are needed therefore to heat the air entering into contact with the catalyst to a temperature of 200.degree. C. It has been found that the use of catalysts according to the patent application EP-A-0 454 531 requires a cumbersome installation, notably because of the heating means, and which is of high energy cost.
The applicant has accordingly sought a process to eliminate substantially at least one of the impurities carbon monoxide and hydrogen without the drawbacks recited above.